The present invention relates generally to differential amplifier circuits, and more particularly relates to a differential amplifier having a low and controllable output common mode voltage.
Differential amplifiers or gain stages are widely used today in many applications, such as high-speed output drivers. A basic function of a differential amplifier, in an idealized form, is to amplify only a difference between two input signals regardless of the common mode value of the signals. An electrical circuit diagram of a simple conventional differential amplifier 100, comprising a pair of bipolar junction transistors (BJT) Q1 and Q2 as an input stage, is shown in FIG. 1A. Since the emitters of the two input transistors Q1 and Q2 are connected together, this circuit configuration is often referred to as an emitter-coupled pair or differential pair.
FIG. 1A depicts one simple form of a conventional differential amplifier. The biasing circuit in the common emitter terminal 102, often referred to as the tail current source, can be either a simple resistor, in which case the equivalent current source 108 will be zero, or a transistor current source (shown in FIG. 1B), in which case the circuit represents a Norton equivalent of the transistor. A more detailed discussion of differential amplifiers in general may be found, for example, in the text by Alan B. Grebene entitled xe2x80x9cBipolar and MOS Analog Integrated Circuit Design,xe2x80x9d John Wiley and Sons (1984), which is incorporated herein by reference.
As shown in FIG. 1A, bias levels and gain characteristics of the differential amplifier 100 are primarily dependent on symmetry between the two branches 104, 106 of the circuit. This balanced nature of the differential amplifier makes it an ideal gain stage for integrated circuits, where close matching can be readily achieved using monolithic components. Another advantage of basic differential gain stages is that they can be directly coupled to one another without the need for extensive voltage level shifting and/or interstage coupling capacitors.
An important parameter in many amplifier designs is output common mode voltage. In general, output common mode voltage refers to the DC value upon which an AC output signal is superimposed, which directly affects, among other things, the output voltage swing of the amplifier.
Referring now to FIG. 1B, a conventional differential gain stage 110 is shown having a bias current circuit or source employing a BJT device Q1 and resistor R1, generally used for desensitizing the high transconductance of transistor Q1. A minimum value of the output common mode voltage at nodes OUT and OUTB for the differential amplifier 110 is primarily limited by the tail current source and a collector-emitter voltage (VCE) of the input differential pair Q2 and Q3. Specifically, the minimum output signal level is determined, at least in part, by the voltage drop developed across resistor R1, a minimum collector-emitter voltage (VCE) of the tail current source transistor Q1, and a minimum VCE of differential input transistors Q2 and Q3. This minimum output signal level is necessary to avoid operation in the saturation region of the transistor devices.
Since the differential amplifier 110 generally requires a large current in order to drive large capacitance loads typically encountered in output driver applications, the voltage drop across resistor R1 (VR1) can easily be a few hundred millivolts (mV), typically around 300mV. Moreover, the collector-emitter voltage (VCE, Q1) for the forward active region of operation for a typical bipolar transistor (e.g., Q1) is generally more than a few hundred millivolts, typically around 400 mV. By summing the above voltage drops (e.g., VR1 and VCE, Q1), it can be readily determined that the bias current source requires about 700 mV minimum in this example. Hence, comparatively low output common mode voltage (e.g., 300mV or below) is difficult to achieve using conventional circuit configurations, especially considering recent trends to lower the voltage supply rails in many applications.
With continued reference to the conventional differential amplifier of FIG. 1B, by eliminating the transistor Q1 in the current (e.g., by connecting resistor R1 directly to the common emitter node), the required common mode voltage can be reduced, at least by the collector-emitter voltage of transistor Q1. However, the bias current I1 flowing through bias resistor R1, which establishes the operating point of the amplifier, will be linearly dependent on the voltage across the resistor R1 (e.g., I1=VEE/R1). Consequently, the bias current will be directly dependent upon the input signal presented to the differential amplifier 110. This is undesirable since the differential gain stage requires a constant current level for stable operation.
In order to reduce the output common mode voltage, other alternative conventional differential amplifier configurations may employ an additional negative voltage supply (e.g., below ground), other than the circuit return supply, to generate an output common mode voltage close to ground potential (e.g., zero volts). From a system standpoint, the inclusion of another power domain is very costly.
Accordingly, there exists a need for a circuit biasing arrangement which provides a reduced output common mode voltage of the circuit. Moreover, it would be desirable to provide selective control of the output common mode voltage.
The present invention provides a driver circuit having a minimized and/or controllable output common mode voltage. The output common mode voltage of the driver is reduced by utilizing a passive element as a bias current source included in the circuit, thus eliminating the need for an active device (e.g., transistor) as is conventionally employed. In order to reduce an undesirable effect associated with using only a passive element as the bias current source of the circuit, namely, the output common mode voltage varying proportionally with an input signal presented to the circuit, the invention includes a current biasing arrangement for providing a common mode output voltage which is substantially independent of the input voltage presented to the driver circuit. Hence, the present invention provides a minimized and stable output common mode voltage.
In accordance with one aspect of the invention, a driver circuit having a minimized output common mode voltage comprises a differential amplifier and a control amplifier operatively coupled thereto in a feedback arrangement. The differential amplifier includes a bias circuit having a passive element (e.g., a resistor) for establishing a bias current in the differential amplifier. The control amplifier senses a voltage corresponding to an output voltage of the differential amplifier and operatively adjusts the bias current flowing through the differential amplifier so that the output common mode voltage of the differential amplifier substantially matches a predetermined reference voltage coupled to the control amplifier.
In an illustrative embodiment of the invention, a driver circuit having a minimized and controllable output common mode voltage comprises a differential output amplifier, a common mode amplifier and a control amplifier. The differential output amplifier includes first and second inputs, and first and second outputs forming a differential output of the driver circuit. The common mode amplifier includes first and second inputs forming a differential input of the driver circuit, and first and second outputs, the first and second outputs of the common mode amplifier being coupled to the first and second inputs, respectively, of the differential output amplifier. The control amplifier includes a first input for receiving a reference signal coupled thereto, a second input for receiving a voltage corresponding to an output voltage from the driver circuit, and an output coupled to the common mode amplifier, the control amplifier being operatively connected in a feedback arrangement in accordance with the common mode amplifier and at least partially controlling an output common mode voltage of the driver circuit in response to a difference between the reference signal and the voltage corresponding to an output voltage from the driver.